1. Field
The present disclosure relates to nickel catalysts for reforming hydrocarbons, methods of manufacturing the same, and hydrocarbon reforming processes using the same.
2. Description of the Related Art
Hydrocarbons (such as natural gas, petroleum gas, or the like) may be reformed in the presence of a reforming material (such as carbon dioxide, water vapor, and oxygen) and a catalyst so as to be converted to hydrogen, carbon monoxide, or the like. Such reactions may be used for various purposes, for example, for hydrogen supply in fuel cells, which are considered to be an alternative energy source to fossil fuels. For example, methane in natural gas may produce a hydrogen gas via a carbon dioxide reforming (CDR) reaction represented by the following Reaction Scheme 1 or a combined steam carbon dioxide reforming (CSCDR) reaction represented by the following Reaction Scheme 2.CH4+CO2→2H2+2CO  [Reaction Scheme 1]0.75 CH4+0.25 CO2+0.5 H2O→2H2+CO  [Reaction Scheme 2]
Such reactions are endothermic and require a relatively high temperature for proceeding in a forward direction. In general, the catalyst degradation that occurs during a reaction may be explained by six mechanisms: catalyst poisoning, fouling (i.e., carbon deposition), sintering, vapor formation, inactive phase formation, and crushing/attrition. When the hydrocarbon reforming reaction is performed at a relatively high temperature, such as in Reaction Scheme 1 and Reaction Scheme 2, carbon deposition and sintering are considered to be the main causes of degradation among the above six mechanisms.
Carbon deposition refers to the physical deposition of chemical species, especially carbon from the fluid phase onto the catalytic surface and in the pores. This deposition steadily occurs from the beginning to the end of a reaction, which decreases the reaction sites of the catalyst and interrupts the diffusion of the reaction gas.
On the other hand, sintering is a catalyst degradation mechanism that is induced by heat. When the catalyst is used at a relatively high temperature and undergoes sintering, the catalyst may become aggregated due to the heat so as to grow into relatively large particles. This results in a decrease in the number or the size of the support pores, and a decrease in the interface area of a catalyst/support. Sintering occurs predominantly at the early stage of reaction. Such sintering phenomenon may result in a smaller area of the catalytically active surface and makes it more difficult for the reaction gas to diffuse into the catalytically active site. Moreover, sintering may cause a decreased interface between the catalyst and the support, leading to a lower level of bonding strength therebetween. As a result, the reaction using the catalyst occurs at a lower conversion rate for reaction gases, the internal pressure of a reactor increases, and the durability of the catalyst/support is deteriorated.